Many CATV transmission and distribution systems currently provide bidirectional communications. Typically, they contain bidirectional amplifiers for transmitting signals both from headends to subscribers and from subscribers to headends. FIG. 1 illustrates a bidirectional CATV transmission and distribution system 100 in accordance with the prior art. The CATV system 100 includes a headend 110, a trunk 120, and distribution lines 130 and 132. An amplifier 160 is provided for amplifying downstream signal 170 and upstream signals 180 and 182. Distribution line 130 connects subscriber drops 140 and 142, and distribution line 132 connects subscriber drops 144 and 146. Subscriber drops 140, 142, 144, and 146 connect subscriber station equipment 150, 152, 154, and 156, e.g., TV sets, respectively.
The station equipment 150, 152, 154, and 156 in the prior art includes upstream signal generation means as described in Canadian Patent No. 1,177,558 to Dufresne et al. (hereinafter the '558 patent). As disclosed in the '558 patent, at least one bidirectional amplifier 160 is usually connected in series with the trunk 120 for amplifying downstream signals 170 and upstream signals 180 and 182. In the prior art system, in addition to transmitting upstream signals, a significant amount of noise is passed upstream from the distribution lines, subscriber drops, and station equipment. This noise is typically caused by electronic or RF signals, poor terminal connections, ground currents, power lines and noise carried thereon, etc. Such noise generally arises on the subscriber drops and distribution lines and is subsequently fed into the trunk and headend in the upstream direction. As such, it has been found that bidirectional systems in accordance with the prior art have been unsuccessful because of a major noise gathering problem in the upstream direction. The noise is random and interferes to a prohibitive extent with legitimate signals transmitted upstream from the various subscribers. The noise problem has been referred to in the art as the "funneling effect" because the noise is aggregated and collected and funneled at the headend 110.
Many current CATV transmission and distribution systems attempt to resolve the above problem of noise ingress in the upstream direction. One such system is disclosed in U.S. Pat. No. 5,126,840 to Dufresne et al. (hereinafter '840 patent). FIG. 2 illustrates a bidirectional CATV transmission and distribution system 200 in accordance with the teachings of the '840 patent. The CATV system 200 includes a headend 210, a trunk 220, and distribution lines 230 and 232. Distribution line 230 connects subscriber drops 240 and 242, and distribution line 232 connects subscriber drops 244 and 246. Subscriber drops 240, 242, 244, and 246 connect subscriber station equipment 250, 252, 254, and 256, e.g., TV sets, respectively. The '840 patent further teaches placing narrowband upstream filters 260 and 262 in the distribution lines 230 and 232, respectively, and/or placing a narrowband upstream filter 264 in the trunk 220 for reducing upstream noise gathering. The narrowband upstream filters 260, 262, and 264 sense upstream signal energy and open when the signal energy exceeds a predetermined threshold.
According to one embodiment of the '840 patent and referring to FIG. 3, upstream signals are contained in one or more narrow bands within the low band 310 preferably centered at two frequencies, 11 MHz, as illustrated by reference number 320, and 26 MHz, as illustrated by reference number 330, with a bandwidth of 1 MHz. Narrowband upstream filters are located in the distribution lines 230 and 232. A result of this technique is that upstream signals outside the narrow bandwidth of the upstream signaling bands are blocked. According to this technique, the likelihood of overloading the upstream amplifiers, e.g., amplifier 270 in FIG. 2, by noise signals is low.
A disadvantage of this technique, however, is that it attempts to prevent noise from entering the CATV system at the amplifier level (e.g., amplifier 270), where hundreds of subscribers are typically connected for funneling upstream signals. As such, the likelihood of an amplifier being switched "on" is very high, i.e., at least one of the many subscribers connected to that amplifier is always transmitting upstream signals at any given time. In other words, since there are so many subscribers connected to each amplifier, the probability of no one using that amplifier and, thus, turning it "off", is practically zero because someone is always transmitting upstream signals. As a result of placing filters at the amplifier level, the CATV system in accordance with the prior art is still susceptible to random noise entering the system even when the majority of the subscribers are not transmitting upstream signals, as long as at least one subscriber is transmitting upstream signals. Moreover, with the switch placed at the amplifier level, if the switch ever fails, then the hundreds of subscribers connected to that amplifier will lose their CATV services. In addition, the more upstream signals funneling into the amplifier, the greater the possibility of false triggering of the amplifier. This is due to the problem of noise compounding from the many subscribers. Accordingly, there is a significant need for an improved apparatus and method for minimizing the problem of reversed noise ingress by terminating reverse paths that have no signal traffic.